Transparent plastic sheets are particularly valuable. For instance, poly(methyl methacrylate) (PMMA) and polycarbonate (PC) sheets are widely used in windows and glass lenses. However, PMMA and PC sheets are expensive. High density polyethylene (HDPE) sheets, like PMMA and PC sheets, have high physical strength, rigidity, and impact resistance. But HDPE sheets are translucent and therefore they cannot be used in windows.
JP 10323892 discloses a method for improving transparency and mechanical strength of polyolefin sheets. The method involves biaxially stretching an HDPE sheet and subsequently compressing the sheet at a temperature within the range of 25° C. below the melting point of the HDPE to 5° C. above the melting point of the HDPE.
JP 53130768 discloses HDPE cups. An HDPE resin is extruded at 210° C. into a sheet of 400 mm wide and approximately 0.9 mm thick, drawn 200% at 122±0.5° C. to give a sheet of 305μ thick and molded to give a cup with Haze value of 16-22%.
JP 51034956 discloses a transparent polyethylene container. A polyethylene sheet containing 70-99.9% of HDPE and 0.1-30% LDPE is heated to a temperature within the range of 5° C. to 35° C. below the melting point of the HDPE and molded with a plug at a temperature below the melting point of the HDPE at a rate less than 500 mm/sec to a draw ratio of 5-15 to give a transparent container.
Although many efforts have been made to increase the transparency of HDPE sheets, highly transparent HDPE sheets which can be used for windows are not available.